The answer to this really depends on how far your system comes up successfully, 
but from what I believe would happen, once you've changed lilo.conf file from 
the MacOS and reboot, your system should come up as far as doing fsck without 
error.

If this happens fsck will fail and bounce you into single user mode to "fix" 
the file system (in this case the problem is that its searching for the / file 
sytem at the old SCSI id).  What you need to do is modify the /etc/fstab file 
to reflect the new id.  Unfortunately, at this point your root file system is 
mounted read-only.  You should be able to do something like:

mount -nw -o remount /dev/sdXX /

where XX is replaced to form the correct new device (not sure if this is 
exactly right command line since I'm not in front of my Mac to double-check the 
command line arguments).  This will remount the / filesystem read/write.  You 
can then edit /etc/fstab to reflect the new location.  When you exit the shell, 
the machine should reboot successfully.  Be sure to change any other lingering 
/dev/sdXX references to the new location although I think fstab will be enough 
to get you up and running.

From a message on mklinux-setup by Mark W. Giles,
copied here by jonh@cs.dartmouth.edu.

I can confirm that this works. I installed DR3 on a 2GB external SCSI using
a PowerPC 9600/200 where the external drive was /dev/sdb. I was able to use
the installed DR3 on the external drive on a new PowerBook G3 as follows.
Changing ROOTDEV from the MkLinux control panel to /dev/sda5 allowed the
powerbook to boot as far as getting a root filesystem readonly. Mounting
with mount -nw -o /dev/sda5 /    allowed the root filesystem to come up
readwrite and editing /etc/fstab restored the system.